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FOT vs. Evaporator size

cgeorge on Mon July 08, 2013 3:32 PM User is offline

Year: 1978
Make: Chevropet
Model: Malibu
Engine Size: 350
Refrigerant Type: R134a
Ambient Temp: 90's
Pressure Low: 50
Pressure High: 250
Country of Origin: United States

I've been dealing with a real head scratcher for a while now on my 1978 Malibu. It never had factory A/C so I purchased an original 1978 A/C plenum assembly from a used parts source and replaced the factory-installed heater plenum. All the inside ducts, vacuum motors, controls, switches, etc. were added or replaced to essentially replicate the factory A/C installation. The heater core, evaporator, and blower motor were replaced with new aftermarket parts from a local auto parts store. The condenser is a Vintage Air parallel-flow condenser, and all new lines were fabricated using Aeroquip fittings and hose. The compressor is a Sanden 7176. The system has been evacuated and changed (many times!) by a local mechanic with solid auto A/C experience.

The only major difference from the factory setup (aside from compressor and condenser) is I used a receiver/drier between the condenser and the orifice tube. According to a local A/C rebuilding shop, either one would do the trick so I went for what was more convenient. A "trinary" pressure switch is installed on the drier to independently control compressor cycling and condenser (radiator) fan activation.

Now for the problem: The system just will not blow cold. It blows cool air, but never really blows below 65-70 degrees (system on Max A/C and confirmed drawing inside passenger compartment air).

We've evacuated for 45 minutes or longer, varied the R134a from less than a pound to over 3 pounds (the original system spec used somewhere around 50 oz. of R12) and tried many FOTs (from .072" GM white down to .047 Ford brown) as well as several different VOTs. What we see is when the pressures look perfect, the evaporator is not cold. Increasing the amount of refrigerant will bring the evaporator temperature down to the point of feeling cold, but the exit air temperature never drops below 65 degrees. Worse yet, as more refrigerant is added, the low side pressure keeps climbing (up to the point where the high side is over 300 and the low side around 85-90).

The thinking around decreasing the FOT orifice size was that it would restrict the refrigerant flow and cause a bigger pressure drop for the low side.

All the obvious things have been checked (amount of PAG oil, flushing condenser and evaporator, damper motors, temperature control door, seals, etc.).

So now to some specific questions that I hope someone here can help provide some insight on.

1. If too small a FOT is used, can it affect the cooling capacity of the evaporator?
2. Is our thinking backwards (i.e. smaller FOT actually increases low side pressures so we should be using a larger FOT)?
3. Does the usage of a drier vs. an accumulator affect the operation of a system with an orifice tube?
4. Would we be better off dumping the FOT and installing a TXV (or is that asking for even more trouble on this custom system)?
5. Is the Sanden 7176 undersized for the system (parallel flow condenser and large OEM evaporator)?
6. Could this just be simply a matter of too much engine compartment heat (the blower, evaporator, heater core, and majority of the plenum project into the engine compartment) and any cooling by the evaporator is being negated by heat transfer through the plastic/fiberglass plenum box walls)?

Any assistance at all will be greatly appreciated. As I stated, the mechanic helping me with this is no slacker. A good portion of his business is fixing auto A/C and his reputation is solid and well-respected. While I fully recognize that diagnosing this problem with a simple post here is a long shot, I am hoping someone out there has encountered a similar situation and can hopefully provide some insight.


iceman2555 on Mon July 08, 2013 10:20 PM User is offlineView users profile


1.) Yes, not sure why it is felt necessary to change from the OE orifice tube. Changing the size drastically can effect the charge level in the evap and thus effect cooling. Never encountered a GM product that would not cool with the correct tube installed. Retro or 134a system.

2.) A smaller diameter orifice tube will produce a increased pressure drop and thus should produce a slightly cooler orifice tube outlet temp...however, a drastic decrease in size produces an increase of pressure/temperature of the inlet side of the orifice and thus offset the temperature change. See #1 above.

3.) A Rec/Drier or Accumulator are not interchangeable. Yes, they do offer some of the same functions, but they also perform other functions that prevent them from being interchangeable. If the system is properly charge and functioning as it should, the failure to use an accumulator may result in serious compressor damage due to ingestion of liquid/vaporous refrigerant into the suction side of the compressor. An accumulator allows for the change of state of the refrigerant as it exits the evap and prior to ingestion into the compressor. The accumulator is also a reservoir for lubricant for the compressor.

4.) The OT system should function well for this system...if it is charged properly....cooled properly. It worked for the OE's and should work for you.

5.) Personally believe the compressor is undersized for this vehicle. Would prefer the use of the SD7H15 unit. Many years ago replaced a York type compressor on a truck with a Sanden 508 and could not make the darn thing cool....TXV system/OE.....made a simple change to the Sanden 510 and back to proper cooling. The other 'ice' may have more to say about this concern.

6.) Not sure about the excessive heat from the engine compartment. If OE parts were utilized, it is doubtful if this is indeed the problem. Insure that all connector locations are intact and there is not possibility of high temp air ingestion into the air box...engine side or passenger side. Seal connectors with AC tape if necessary. Insure there is not any high air temp bleed from the heater core area.

First thoughts would be the possible down size of the compressor and a possible refrigerant flow restriction of the Rec/Drier could reduce the amount of refrigerant in the evap and thus prevent adequate cooling performance. A HPCO switch on the Rec/Drier does not cycle the compressor in the same manner as a LPCO switch on the accumulator. The HPCO cycles due to excessive high side pressures and the LPCO cycles to prevent evaporator freeze up.

Think the system should be plumbed as designed....replace the Rec/Drier with the correct accumulator and install the GM white orifice tube....should be in the inlet of the evaporator (bottom line). Charge until the evap INLET and OUTLETs are the same temp or within 3-5 degrees of each other. Perform this test with the DOORS OPEN....ENGINE @ IDLE....MAX COOL...HIGH BLOWER. Compare evap outlet temp to center vent for a drastic temp results.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

cgeorge on Tue July 09, 2013 1:20 PM User is offline

Thanks for the quick and detailed response. For various reasons, it will be a few weeks until I can actually try anything bu I will be sure to post results here.

Regarding the pressure switch, I should have mentioned that the car has a thermostat to cut out the compressor to prevent freeze up (i.e. 12v -> NC thermostat -> NC HPCO switch -> compressor clutch). Based upon your statement, the trinary switch on the drier can not be used on the accumulator, correct? I would need a LPCO switch for the accumulator and a separate switch on the high pressure side to control the fan (back in '78, the car had a mechanical fan so there was no fan switch, so I'd have to run the +12v A/C control directly to the fan relay, thus running the fan 100% of the time while the A/C is on). Or will the same switch work because it is only on for a pressure range (28-377 psi)?

I really hope the 7176 will work out. The serpentine system mounts the compressor below the alternator, and there is limited space between the bracket and block, so the 508 style compressor would be too deep.


cgeorge on Mon September 02, 2013 7:34 PM User is offline

Hi Iceman, I know it's been a while but I finally reworked the system and have some data.

I removed the drier and installed an accumulator on the evap outlet. The #6 hose from the condenser now flows through a pressure switch fitting and a #6 to #8 adapter connected to the evap inlet, and the accumulator outlet connects through an adapter to the #10 hose to the compressor inlet. To start things out we used a standard white GM orifice (0.072").

THe system was instrumented with 4 temperature probes, one on the evap inlet pipe, one on the evap outlet pipe, one in the air plenum inlet, and one in the center dash vents.

We evacuated and charged the system initially with 1 lb. R134a and started the engine. The best cooling obtained was with 2 lb 8 oz refrigerant. This yielded about 65 degrees on both sides of the evap core and the air exit temperature was about 78 degrees (hot day, inlet temperature was 95). Compressor was cycling with a trinary switch, so pressure would build to 350 then fan cycle on, dropping hi pressure to 250. Low side was riding 70 to 90 psi.

We evacuated and replaced the orifice tube with an orange (0.057") because that was all we had handy. Charged the system with 1 lb and started the engine. Immediately say evap inlet temp drop to 44 degrees but the outlet was still high. Brought refrigerant up to 2 lb 8 oz a couple of oz at a time and now saw evap temps drop into the mid 50's with low side pressure around 60. So obviously dropping the orifice size made a difference.

I drove it for a few days and while it cools, it never really chills. The weather climbed into the 100s last week so there was opportunity for good system testing. Obviously, the system cools best in the mornings when the ambient and underhood temperatures are at their lowest. After sitting all day in the sun it does cool, but the whole sitting in traffic thing is really straining it. I also noted that the system works better with the radiator fan on always rather than having the trinary switch control it.

Keep in mind the system never works this well, so the accumulator was a step in the right direction.

So when I got one the other afternoon I made some measurements:

Ambient temperature: 96
Underhood surface temperature: 115
Static refrigerant pressure: 120 psi
(started engine)
(Radiator fan always on)

Blower speed: Med
Evap Inlet: 56
Evap Outlet: 59
Air Inlet: 88
Air Outlet: 67
Lo PSI: 58
Hi PSI: 250

Blower speed: Hi
Evap Inlet: 61
Evap Outlet: 64
Air Inlet: 86
Air Outlet: 72
Lo PSI: 68
Hi PSI: 270

So any thoughts?


mk378 on Tue September 03, 2013 12:21 AM User is offline

You don't have enough compressor for the amount of heat reaching the evaporator. The low side has to get lower to drop the boiling point and (potentially) get the air colder. Smaller OT may help some, but really it looks like the compressor just isnt keeping up.

Those pressures were at idle I assume. What happens when you rev it?

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